1. Field of the Invention
The present invention relates to an optical information recording/reproduction method and apparatus for optically recording/reproducing information on/from a card-shaped recording medium.
2. Related Background Art
Conventionally, a recording medium on which information is optically recorded or from which recorded information is optically read out, can take on various shapes such as a disk shape, a card shape, and the like are known. Of these information recording media, a card-shaped recording medium (to be referred to as an optical card hereinafter) is expected to be in great demand in the future since it is compact, lightweight, and convenient to carry, and has a large recording capacity.
When information is recorded on a recording medium such as an optical card, a light beam, which is modulated in accordance with recording information and is focused to a very small light spot, is scanned on an information track thereof, thereby recording the information as an optically detectable recorded pit string. Upon recording of information, in order to accurately record information without causing trouble such as crossing an information track, auto-tracking control is performed to control the irradiation position of the light beam in the track intersecting direction, so that the light beam is scanned to follow the information track. Of course, upon reproduction of information, auto-tracking control is performed to control the light beam to be scanned to follow the information track.
FIG. 1 is a diagram showing an example of an optical card recording/reproduction apparatus for recording/reproducing information on/from an optical card. Referring to FIG. 1, an optical card 100 as an information recording medium is placed on a shuttle 101. The shuttle 101 is movable in the track intersecting direction by driving an ultrasonic wave motor (to be abbreviated as a USM hereinafter) 103. As will be described later, correction control with respect to a skew of the optical card 100 is realized by moving a light beam in the track intersecting direction under the control of a USM control circuit 104. A semiconductor laser 105 serves as a recording/reproduction light source, and a collimator lens 106 collimates a light beam emitted from the semiconductor laser 105.
A light beam emitted from the semiconductor laser 105 is incident on an objective lens 108 via the collimator lens 106 and a polarization beam splitter 107, and is focused to a very small light spot by the objective lens 108. The optical card 100 is irradiated with the small light spot. The irradiated light is reflected by the surface of the optical card 100, and the reflected light is converted into collimated light again via the objective lens 108. The collimated light is transmitted through the polarization beam splitter 107, and is guided to a polarization beam splitter 109. The collimated light is reflected by the polarization beam splitter 109, and is focused by a focusing lens 110. The focused light is received by a photoelectric conversion element 111 for tracking control. The above-mentioned optical elements such as the semiconductor laser 105, the objective lens 108, the photoelectric conversion element 111, and the like are integrated as an optical head 102, and the optical head 102 is reciprocally movable in the track direction of the optical card 101. Therefore, upon reciprocal movement of the optical head 102, the light beam output from the optical head 102 is reciprocally moved relative to the optical card 100, and the light beam scans an information track. Note that the optical head 102 may be fixed in the track direction, and the shuttle 101 may be reciprocally moved in the track direction.
The light-receiving signal of the photoelectric conversion element 111 is input to a tracking control circuit 112, and the tracking control circuit 112 generates a tracking error signal indicating a shift amount and a shift direction of the light beam with respect to the track on the basis of the light-receiving signal. The tracking control circuit 112 drives a tracking actuator 113 on the basis of the tracking error signal to finely move the objective lens 108 in the track intersecting direction, thus performing the tracking control, so that the light beam does not fall outside the information track upon scanning of the light beam, as described above. In this manner, the tracking control of the light beam is performed, and information is recorded or reproduced under the tracking control upon recording or reproduction of information.
Note that each information track of the optical card 100 is often not parallel to the track direction but is slightly inclined at a skew angle .theta., as shown in FIG. 2. When a light beam is scanned on the optical card with a skew under the tracking control, since the movable range of the objective lens 108 is limited, the light beam sometimes falls outside the control range of the tracking control, and cannot follow the information track. In such a case, correction control of the optical card 100 with respect to a skew is performed by moving the optical card 100 in the track intersecting direction by the USM 103. More specifically, assume that the movable range of the objective lens 108 is a .+-.25-.mu.m range on each of the right and left sides of the lens center, and the light beam from the optical head 102 is scanned on an information track of the optical card, which has the skew angle .theta., as shown in FIG. 2. At this time, even when the optical card is skewed, the objective lens 108 is driven in correspondence with the skew under the tracking control as long as the information track is present within the movable range of the objective lens 108. For this reason, the optical beam is scanned to track the information track.
On the other hand, when the objective lens 108 moves to the right or left from the central position of the optical head 102 by 25 .mu.m as the movable range, the tracking control circuit 112 outputs a control signal to the USM control circuit 104 to move the shuttle 101, so that the objective lens 108 returns to the central position of the optical head 102. In response to this signal, the USM control circuit 104 controls the USM 103 to move the shuttle 101 in the track intersecting direction. In this state, since the tracking control is active, the objective lens 108 moves toward the center of the optical head to follow the shuttle 101. In this case, the USM control circuit 104 outputs two-phase sine wave drive signals having a 90.degree. phase difference therebetween to drive the USM 103. When the objective lens 108 reaches the center of the optical head 102, i.e., the center of the movable range, the tracking control circuit 112 outputs a control signal to the USM control circuit 104 to stop the shuttle 101. In this manner, when the optical card 100 is skewed, the objective lens 108 is controlled to return to the center of the optical head 102 by moving the shuttle 101, and the correction control for the positional relationship between the light beam and the information track is performed.
However, the conventional information recording/reproduction apparatus uses the USM as drive means for moving the shuttle in the track intersecting direction. When a means with high response characteristics such as a USM is used, the shuttle is abruptly accelerated upon the start of the drive operation of the USM, and is abruptly decelerated upon stopping of the drive operation. For this reason, the tracking error amount (AT error amount) undesirably increases upon starting and stopping of the drive operation of the USM, and if the AT error amount is too large, overrun may occur or failure of AT control may occur. In particular, a change in the tracking error amount upon stopping of the driving operation is large, and AT control failure upon stopping of the driving operation poses a serious problem.